Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/1995
Publication Date: N/A
Citation: Interpretive Summary: Growth is routinely measured as the change in live weight, but is defined as the combined growth of the major tissues, primarily protein, fat and bone. Fill often complicates accurate measures of animal weight and growth since fill is not composed of animal tissue, but of food ingesta and water. Body fat percentage seems to be constant at a given proportion of mature body size, which we define as the maximum body protein mass. Protein growth declines to zero when cattle reach their mature body size, but mature cattle can continue to accumulate fat. Mature body size occurs at about 36 percent empty body fat in modern cattle. Efficiency of fat accumulation is 1.7 times that of protein on an energy basis. However, on a weight basis lean tissue gain is four times more efficient than fat because water is deposited with protein. Conversion of protein to fat is very inefficient.
Technical Abstract: Growth in animals is defined as accretion of protein, fat, and bone. Although growth typically is measured as the change in live weight, nutrient retention is estimated more precisely by measuring empty body weight and composition, whereas production economics are measured ideally through carcass weights and quality. As a percentage of live weight gain, carcass weight gain usually is a much higher percentage during the feedlot phase than during the growing phase of production because dressing percentage (ratio of carcass:live weight) increases with maturation and is greater with concentrate than with roughage diets. At a given fraction of mature body size (maximum body protein mass), body fat percentage seems to be a constant. Mature size may be altered genetically and nutritionally. Protein accretion declines to zero when cattle reach their mature body size (approximately 36% fat in empty body weight in modern cattle) even though mature animals can continue to accrete fat. Although fat accretion can be reduced by limiting the supply of net energy, rate of fat accretion by finishing steers given ad libitum access to high-concentrate diets seems to reach a plateau at approximately 550 g daily. Protein mass, in contrast, increases in proportion to empty body weight. Energetically, efficiency of accretion of fat is approximately 1.7 times that of protein. But because more water is stored with deposited protein that with deposited fat, lean tissue gain is four times as efficient as accretion of fat tissue. Conversion of protein to fat is very inefficient, suggesting that excess protein is utilized inefficiently.